This paper discusses a multi-threaded software architecture for Message-Passing Interface (MPI) software specification. The architecture is thread-safe, allows for concurrent communication over several communications media (multi-fabric communication), efficiently utilizes available hardware concurrency over a wide range of target platforms, and allows for concurrent communication and computation within the limits imposed by the hardware. The architecture is developed in the framework of the MPICH software architecture, a well-known MPI implementation used worldwide. The proposed architecture adopts wide portability of the MPICH design and remedies some of its deficiencies such as inefficient multifabric communication and non-thread-safety. The paper also considers the issues concerning development of high-performance portable message-passing systems for general-purpose architectures. The contributions of the paper are: improving architecture and addressing thread safety of modern reli...